the last stable version (3.0.4.6).
In /usr/local/ispconfig/interface/admin/iptables*
In /usr/local/ispconfig/interface/admin/templates/iptables*
In /usr/local/ispconfig/interface/admin/list/iptables*
...
Link this are comment in /usr/local/ispconfig/interface/admin/lib/module.conf.php

This is a unfinished approach which has been abandoned. The files will get removed. Starting with ispconfg 3.0.5, ispconfig supports ufw were you can configure custom rules which are not supported by the ispconfig interface with the ufw command(s).